1. Field of the Invention
The present invention relates to an article with antifouling properties for use in aquatic applications, in particular marine applications, and also to a process for retarding the growth of aquatic organisms on submersible or semi-submersible structures.
2. Description of Related Art
The invention relates to the field of antifouling marine paints. Antifouling marine paints are top coats for preventing the attachment of animals or plants to ship hulls. They are used for safety reasons, for maintaining the manouverability of ships, for reducing the consumption of fuel, and for combating corrosion and the emburdening of structures.
The problem of “biofouling” is a major problem resulting from the submersion of materials into marine environments. A considerable maintenance cost is involved in preventing this phenomenon.
Specifically, the formation of “biofouling” or “fouling” takes place during submersion in seawater, in which a layer of organic and mineral molecules is adsorbed onto the surface of the material extremely rapidly. This layer of adsorbed material or “biofilm” serves as a mediator for the adhesion of the bacteria present in suspension in the marine environment.
This surface colonization by marine bacteria is rapid and a stationary state is reached after a period of a few hours to a few days. Finally, other marine organisms come to colonize the surface, the adherent bacteria recruiting these other organisms. This set of live organisms connected to the surface constitutes the “biofouling” or “fouling”.
The adhesion of marine fouling concerns any structure submerged in the sea: ships, pipelines, cooling towers and circuits, port structures, marine sensors, aquaculture systems, etc. Diverse and extensive damage is thereby caused. Specifically, these structures become encrusted, for example, with organisms that have a negative effect on the performance qualities of the structures.
In particular, for ship hulls, the encrustation of various marine organisms increases the friction between the ship hulls and the seawater, which reduces the speed and may lead to greater fuel consumption. Thus, the bottom of a ship that is not protected with an antifouling system may, after less than six months spent at sea, be covered with 150 kg of fouling per square meter.
To avoid this economic loss, and also to better inhibit corrosion, paints known as antifouling paints are applied to the submerged parts of structures exposed to water, the purpose of these paints being to prevent or to notably reduce the encrustation fouling of marine organisms. The principle of antifouling paints is based on the controlled release of the active substance at the interface between the surface and the seawater. The efficacy of the paint is maintained as long as the concentration of active substance released at the surface is efficient and regular. The majority of antifouling paints thus contain a biocidal product which destroys marine organisms, which is usually an organometallic compound (based on tin, copper or zinc) or an organic compound (fungicide, algicide or bactericide) which prevents the adhesion of marine fouling via their toxic activity.
However, the problem associated with the use of these paints is that they release into the marine environment substances that are harmful to marine fauna and flora. Furthermore, the coatings become increasingly coarse by gradually degrading, which increases the fuel consumption and augments the hydrodynamic noise emitted by the submerged structure.
This novel difficulty has been solved by using self-cleaning antifouling paints. In addition to containing biocidal agents, these paints have, under the action of surface hydrolysis by seawater and that of erosion due to the movement of the ship, a regular and controlled loss of thickness over time. The slow erosion of the coating on contact with seawater allows the surface to be constantly renewed with biocidal agents.
The self-cleaning antifouling paints developed since the 1960s were based on tin salts. These are self-cleaning paints formulated with tributyltin (TBT) methacrylate copolymers which have a constant degree of leaching. The TBT grafted onto an acrylic binder is released slowly by hydrolysis in water. Examples of this type of paint are described in documents FR-A-2 266 733, FR-A-2 557 585, EP-A-0 051 930 and GB-A-2 118 196.
Tributyltin (TBT), which is very efficient, was thus the biocide most commonly used in antifouling paints, but this product, its degradation molecules and its metabolites have proven to be seriously and durably polluting. For these reasons, the International Maritime Organization has banned the use of tin-based antifouling paints.
The antifouling paints now used are mainly based on copper compounds and/or synthetic chemical compounds, but also based on polymers of silicone type.
For the copper-based paints, although they are less toxic than tin salts, they are virtually always formulated with a large proportion of cuprous oxide (see, for example, document EP-A-051 930 or FR-A-2 557 585), the main binder being based on special polymers generally of acrylic type. However, they are only effective against marine fauna, and, to combat the growth of algae, it is essential to add herbicides, which may place new threats on the environment.
This alternative therefore does not provide a durable solution for protecting the environment from the massive discharge of heavy ions, especially copper ions, following the intensive use of tin-free but copper-rich paints.
Another solution for preventing the fouling of the surfaces of structures in contact with seawater consists in covering these surfaces with at least one protective coating, the outer coat of the coating in contact with the water being a silicone elastomer. These coatings are prepared from paints known as “fouling-release coating” paints. The principle of these novel antifouling paints is to create a very smooth surface with a low surface energy, to which organisms have great difficulty in adhering. When a ship has stopped, these surfaces are stationary and marine organisms can be deposited thereon. However, by virtue of the suppleness and the low surface tension of the silicone-based top coat, these organisms are simply removed by the force of the movement of water or the effect of friction caused by the movement of the ship. This also means that if there is sufficient movement of water about the hull of a ship, a natural self-cleaning effect takes place.
By virtue of these properties, even ships that are less frequently at sea or in waters with less movement benefit from more spaced apart cleaning intervals. This is due to the fact that marine organisms have difficulty in adhering to the surface, which makes the cleaning easier.
These silicone-based paints forming an antifouling coating are thus very innovative:                they are totally friendly to the marine environment: no discharge of metals, and        they improve the slippage of ships, thus reducing their fuel consumption by 1% to 5% and thus their emissions of greenhouse gases.        
Many patents, for example patents FR-A-2 083 029 and U.S. Pat. No. 3,702,778, describe such coatings whose final coat, known as the “top coat”, is made of hot-cured or cold-cured silicone elastomer.
For example, patent application U.S. Ser. No. 07/847,401, filed on Mar. 6, 1992, discloses an antifouling system containing three components, comprising at least one coat of an epoxy primer, an adhesion primer or fixing coat (tie coat) and an antifouling coat (top coat) based on silicone elastomer. The final coat of epoxy primer is normally a thin coat that is applied to obtain a clean and fresh surface onto which the tie coat can adhere. The tie coat comprises an organopolysiloxane and a curing constituent. The antifouling coat comprises an organopolysiloxane, an alkyl silicate and a curing agent or a separate tin-based catalyst. The coat(s) of epoxy primer are applied directly onto the support. The tie coat is applied onto the coat(s) of epoxy primer. The antifouling coat of silicone coating is then applied and crosslinked on the tie coat, after partial curling of the latter.
An antifouling coat (top coat) based on silicone elastomer may also comprise fluids that improve the “antifouling” effect, in particular:                methylphenylpolysiloxane oils (U.S. Pat. No. 4,025,693),        a hydrocarbon-based liquid compound, for example a polyolefin,        a plasticizer,        a lubricant oil (FR-A-2 375 305),        liquid paraffins and waxy masses such as petrolatum (JP-A-83/013 673),        a thermoplastic polymer such as PVC,        a vinyl chloride/vinyl acetate copolymer (Kokai JP-A-79/026 826), or        cationic, anionic, nonionic or amphoteric surfactants (JP-A-85/258 271).        
In order to form the silicone elastomer coating, the silicone formulations used generally involve a silicone oil, generally a reactive polydimethylsiloxane bearing hydroxyl end groups, optionally prefunctionalized with a silane so as to have alkoxy end groups, a crosslinking agent and a polycondensation catalyst, conventionally a tin salt or an alkyl titanate, a reinforcing filler and other optional additives such as packing fillers, adhesion promoters, colorants, etc.
These room-temperature vulcanizable organopolysiloxane compositions are well known and are classified into two distinct groups: one-pack compositions (RTV-1) and two-pack compositions (RTV-2). The term “RTV” is the abbreviation for “room-temperature vulcanizing”.
During crosslinking, water (either provided by atmospheric humidity in the case of RTV-1, or introduced into part of the composition in the case of RTV-2 using catalysts based on tin carboxylate) allows the polycondensation reaction, which leads to the formation of the elastomer network.
Generally, the one-pack compositions (RTV-1) crosslink when they are exposed to atmospheric humidity, i.e. they cannot crosslink in a confined medium. For example, one-pack silicone compositions crosslink without heating according to a mechanism of hydrolysis of reactive functions of the acetoxysilane, ketiminoxysilane, alkoxysilane, etc. type, followed by condensation reactions between formed silanol groups and other residual reactive functions. The hydrolysis is generally carried out by virtue of water vapor which diffuses into the material from the surface exposed to the atmosphere. Generally, the polycondensation reaction kinetics are extremely slow; these reactions are thus catalyzed with a suitable catalyst. As catalysts which are used, use is most often made of catalysts based on tin, titanium, an amine or compositions of these catalysts. Catalysts based on tin (cf. in particular FR-A-2 557 582) and on titanium (cf. in particular FR-A-2 786 497) are catalysts that are very effective. One-pack silicone elastomers bearing —Si(OR) end groups are occasionally referred to as alkoxy elastomers.
As regards the compositions packaged in the form of two-pack products (RTV-2), a first component (or part) comprising compounds that are capable of polycondensing and the second component is airtight and contains the catalyst and usually one or more crosslinking agents. The two components (or parts) are mixed together during use and the mixture cures (via crosslinking reactions) in the form of a relatively hard elastomer, especially when the composition comprises reinforcing fillers. These compositions packaged in two-pack systems are well known and are described, in particular, in the book by Walter Noll “Chemistry and Technology of Silicones” 1968, 2nd Edition, on pages 395 to 398. These compositions usually comprise the following ingredients:                a reactive polydiorganosiloxane with silanol groups at the end of the chain (for example an α,ω-di(hydroxydimethylsilyl)(polydimethylsiloxane), in the chain or at the end of the chain and in the chain,        a crosslinking agent,        a condensation catalyst, and        optionally water, often present when a dialkyltin dicarboxylate is used as catalyst (activation of this catalyst by the presence of water).        
Usually, the condensation catalyst is based on an organic tin compound. Indeed, many tin-based catalysts have already been proposed as a catalyst for crosslinking these RTV-1 or RTV-2 compositions. Conventional polycondensation catalysts comprise dialkyltin compounds, especially dialkyltin dicarboxylates such as dibutyltin dilaurate and diacetate, alkyl titanate compounds such as tetrabutyl or tetraisopropyl titanate, or titanium chelates (EP-A-0 885 933, U.S. Pat. No. 5,519,104, U.S. Pat. No. 4,515,932, U.S. Pat. No. 4,563,498, U.S. Pat. No. 4,528,353).
However, the alkyltin-based catalysts, although very effective, usually colorless, liquid and soluble in silicone oils, have the drawback of being toxic (CMR2 toxic for reproduction).
For a durable development, it thus appears necessary to develop novel antifouling paints not comprising any toxic catalyst. For example, an important characteristic of a curable silicone composition is the working time (pot life or working time), i.e. the time for which the composition can be used after mixing without curing. This time should be long enough to allow its use, but short enough to obtain a hard coating. For example, for a coating of tie coat or top coat type, a working time of between 2 and 4 hours is generally required when the exterior temperature is between 20 and 30° C. Outside this range, one of the means for adjusting this working time is the nature of the components used such as the catalyst or the crosslinking agent.
For all these reasons, novel strategies for combating the adhesion of aquatic fouling and in particular marine fouling are now being developed.